The present invention is a process for the hydrocracking of carbohydrates in the presence of a homogeneous transition metal catalyst to give commercially important glycols.
In the past, conversions of this type have been carried out using heterogeneous metal catalysts at high temperatures, typically about 200.degree. C., with hydrogen pressures over 500 psi, in the presence of promoters. For example, U.S. Pat. No. 4,496,780 disclosed the hydrocracking of carbohydrate polyols using a heterogeneous catalyst of a Group VIII metal such as ruthenium composited on a support and an alkaline earth metal oxide. The process requires a temperature of 150.degree. to 250.degree. C. and a pressure of 500-5,000 psig. U.S. Pat. No. 4,476,331 discloses hydrogenolysing of hydrogenated sugars to lower polyhydric alcohols using a supported, sulfided RuCl.sub.3 catalyst and a base, at a temperature of 150.degree.-300.degree. C. and a hydrocarbon pressure of 500-5,000 psig. U.S. Pat. No. 4,401,823 describes the hydrogenolysis of plyhydroxylated compounds in the presence of a shaped carbonaceous prepolymer impregnated with a Group VIII metal such as ruthenium, at 175.degree.-250.degree. C. and 10-2,000 psi.
Other heterogeneous hydrogenation catalysts such as supported nickel, platinum or palladium used with bases in the hydrogenolysis of carbohydrates are described in U.S. Pat. Nos. 4,404,411 4,380,678, 3,396,199 and 3,030,429. Typical feedstocks are glucose or glucitol (sorbitol) and typical products are mixtures of ethylene glycol, propylene glycol and glycerol.
U.S. Pat. No. 2,209,055 discloses a process for cleaving monosaccharides in the presence of a tertiary amine and a Raney nickel catalyst in a solvent at a temperature of at lest 75.degree. C. and a hydrogen pressure of at least 2,000 psi.
Heterogeneous catalysts form a separate phase from reactants and products and are generally solids. Homogeneous catalysts, in contrast to heterogeneous catalysts, can be dispersed in solution.
Soluble ruthenium complexes have been used as homogeneous catalysts for hydrogenation reactions. Linn, Jr., D. E. et al., J. Am. Chem. Soc., 109, 2969-2974 (1987) discloses the hydrogenation of ketones and arenes in the presence of RuH.sub.4 -(PPh.sub.3).sub.3 or [RuH.sub.3 (PPh.sub.3).sub.3 ].sup.- in tetrahydrofuran solution. U.S. Pat. No. 3,935,284 discloses homogeneous catalytic hydrogenation of sugars in the presence of a ruthenium triphenylphosphine complex and a strong acid. For example, fructose was hydrogenated to yield mannitol and glucitol. However, no carbohydrate hydrocracking has been previously undertaken or suggested using homogeneously catalyzed reactions.
Accordingly, it is an object of the present invention to provide a process for the hydrocracking or hydrogenolysis of carbohydriates using homogeneous catalysts under milder conditions than previously possible with better product selectivity, thereby reducing the costs associated with both substrate conversion and product purification.